The present invention relates to a vinyl chloride resin composition possessing improved processability and excellent mechanical properties. More particularly, the invention relates to a vinyl chloride resin composition comprising a polymer processing aid comprising an alkyl methacrylate copolymer and a copolymer comprising as its essential constituents an alkyl methacrylate, an aromatic vinyl monomer and diolefin.
While vinyl chloride polymers are widely used because of their excellent physical and chemical properties, there remain various disadvantageous problems in their processability, such as a narrow range of processing temperatures at which forming or molding can be carried out due to only a small difference between the processing temperatures and thermal decomposition temperature of the polymer, and also a slow rate of gelation. It is well known that such drawbacks can be eliminated to some extent by the addition of a plasticizer. However, the addition of a plasticizer not only gives rise to other problems such as evaporation or migration of the plasticizer, but also deteriorates the mechanical properties of formed polymer products.
On the other hand, in order to obtain improved processability so that the formed products of the polymer will have a smooth surface, the rate of gelation will be increased, and deep drawing will be successfully carried out, there have been some attempts to add to a vinyl chloride polymer, as a polymer processing aid, a copolymer having compatibility therewith, such as a copolymer of styrene and acrylonitrile, a copolymer predominantly comprising methyl methacrylate and styrene, or a copolymer predominantly comprising methyl methacrylate and an alkyl acrylate.
However, all these prior art methods for improving the processability of vinyl chloride polymers have serious drawbacks as follows.
A copolymer predominantly comprising styrene and acrylonitrile impairs the thermal stability of the polymer, and does not significantly improve its processability such as the gelation rate and deep drawing properties.
In comparison with the copolymer of this type, copolymers predominantly comprising methyl methacrylate and styrene and copolymers predominantly comprising methyl methacrylate and an alkyl acrylate effectively accelerate the gelation rate and also increase the elongation at a high temperature, whereby satisfactory deep drawing properties can be obtained, and adaptability for fabrication, for example, vacuum forming, is markedly improved.
However, on the other hand, these copolymers lower the mechanical strength of a resin composition. In particular, a composition obtained by adding a polymer processing aid as described above to a composition comprising a vinyl chloride polymer and an impact strength modifier such as an acrylonitrile/butadiene/styrene copolymer or a methyl methacrylate/butadiene/styrene copolymer has a notably lowered impact strength.